Changes in the number of neurons in the mesencephalic and motor nuclei of the trigeminal nerve in the ageing mouse brain.

The number of neurons and neuronal nuclear diameter was estimated in the mesencephalic and motor nuclei of the mouse trigeminal nerve at 6, 9, 12, 15, 22, 25, 28 and 31 months of age. Analyses of variance showed that neuronal number decreased significantly (P less than 0.01) in the mesencephalic nucleus between 25 and 31 months, but, although lowest at 31 months of age in the motor nucleus, the decrease in neuronal number in the motor nucleus was not statistically significant. Neuronal nuclear diameter increased significantly in both nuclei between 25 and 31 months, being obvious first in the mesencephalic nucleus. The main histological features of ageing were a marked accumulation of lipofuscin granules in neurons of the mesencephalic nucleus and a loss of Nissl substance from the motor neurons of the trigeminal motor nucleus.